1. Field of the Invention
The present invention relates to a bumper structure of a vehicle provided with a pair of left and right crush cans.
2. Description of the Related Art
A conventionally known bumper structure of a vehicle includes a transversely extending bumper beam attached to forward ends of left and right front side frames, an energy absorbing member made of urethane or the like attached to a front side of the bumper beam and a bumper face disposed in front of the energy absorbing member as described in Japanese Utility Model Publication No. 1994-57751, for example.
In today's automotive industry, however, it is becoming a mainstream trend for a vehicle to employ a bumper structure including a pair of left and right crush cans attached to forward ends of left and right front side frames, a bumper beam attached to forward ends of the left and right crush cans, an energy absorbing member and a bumper face, the energy absorbing member and the bumper face being constructed essentially the same way as in the aforementioned conventional bumper structure. The crush cans employed in this new type of bumper structure deform compressively, or crush, due to an impact force produced in the event of a vehicle collision, thereby absorbing impact energy in an effective fashion.
Generally, there are two types of frontal collisions. These are full-wrap collisions in which an obstacle, such as another vehicle, collides with own vehicle's bumper structure from a front direction, involving almost an entire bumper width, and offset collisions in which an obstacle collides with the bumper structure, involving part (left or right side) of the bumper width. In the case of a full-wrap collision, the impact force is divided approximately into halves and exerted almost equally on the two crush cans whereas, in the case of an offset collision, the impact force concentrates on one of the two crush cans.
A more recent requirement in bumper design is for a bumper structure which is constructed in such a way that an energy absorbing member and a bumper face would warp (deform) and absorb impact energy without causing a pair of left and right crush cans to crush in the event of a low-speed full-wrap collision, so that cost and time needed for repairing the damaged crush cans would be reduced. It would be possible to meet this requirement by designing the crush cans to have an increased overall stiffness in a longitudinal direction, for example.
If the overall longitudinal stiffness of the crush cans is increased, however, neither of the crush cans will crush enough in the event of an offset collision, especially in a low-speed offset collision, thus creating a possibility that the impact energy might not be sufficiently absorbed. To be more specific, the impact energy produced by an offset collision can hardly be absorbed by warpage of the energy absorbing member and the bumper face and would be exerted on one of the two crush cans in a concentrated fashion. If the stiffness of the crush can is too high, the crush can will not crush sufficiently and, as a consequence, the impact force will be transmitted directly to one of the front side frames. This will cause a damage to the front side frame and an eventual increase in repairing cost.
If the overall longitudinal stiffness of the crush cans is reduced to cope with the aforementioned problem, one of the crush cans will crush in the event of a low-speed offset collision, thereby absorbing the impact force effectively. If the crush cans are so constructed, however, both of the crush cans will crush even in the event of a low-speed full-wrap collision. Then, it will become impossible to satisfy the aforementioned requirement for the bumper structure whose energy absorbing member and bumper face would warp and absorb the impact energy without causing the pair of crush cans to crush to permit a reduction in repairing cost caused by the low-speed full-wrap collision.